Process for producing maleic anhydride

ABSTRACT

Maleic anhydride is produced by the oxidation of a nonaromatic hydrocarbon having at least four carbon atoms in a straight chain with molecular oxygen or a molecular oxygen-containing gas in the vapor phase in the presence of a phosphorus-vanadium mixed oxide catalyst. Such catalysts are prepared by contacting a tetravalent vanadium compound, dissolved in an aqueous, non-oxidizing acid medium, with crystalline diphosphoric acid to form a phosphorus-vanadium mixed oxide catalyst precursor. The resulting catalyst precursor-containing solution is subjected to a series of concentration/dilution cycles to induce crystallization of the catalyst precursor. The crystals are collected, dried, formed into desired structures, and calcined at temperatures from about 300° C. to about 600° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the production of maleicanhydride by the oxidation of nonaromatic hydrocarbons. Moreparticularly, this invention is directed to a process for the partialoxidation of nonaromatic hydrocarbons in the vapor phase with molecularoxygen or a molecular oxygen-containing gas to produce maleic anhydridein excellent yields.

Maleic anhydride is of significant commercial interest throughout theworld. It is used alone or in combination with other acids in themanufacture of alkyd and polyester resins. It is also a versatileintermediate for chemical synthesis. Significant quantities of maleicanhydride are produced each year to satisfy these varied needs.

2. Description of the Prior Art

The preparation of phosphorus-vanadium mixed oxides and their use ascatalysts in the production of maleic anhydride is known. As an example,U.S. Pat. No. 4,085,122 discloses a process which involves a salt oftetravalent vanadium, dissolved in an aqueous, non-oxidizing acidsolution, with orthophosphoric acid. After the solution is concentrated,the vanadium salt complex is pre-cipitated by adding water. Theprecipitate is collected, dried, formed into the desired form, andsubjected to a heat treatment of at least 300° C. to produce thecatalyst. The resulting catalyst is concontacted with a C₄ hydrocarbon,n-butane being preferred, in the vapor phase at a temperature between320° C. and 500° C. to yield maleci anhydride.

U.S. Pat. No. 3,293,268 teaches a process of oxidizing saturatedaliphatic hydrocarbons to maleic anhydride under controlled temperatureconditions in the presence of a phosphorus-vanadium mixed oxide catalystproduced in a specified manner.

Although these prior art processes generally produce the desired maleicanhydride product, the commercial utility of a catalytic process ishighly dependent upon the cost of the catalyst employed, the conversionof the reactant(s), and the yield of the desired product(s). In manyinstances, a reduction in the costs of a catalyst system employed in agiven process on the order of a few cents per kilogram or pound, or asmall percent increase in the yield of the desired product represents atremendous commercial economical savings and advantage. Accordingly,research efforts are continually being made to define new or improvedcatalyst systems and methods and process of making new and old catalystsystems to reduce the costs and/or upgrade the activity and selectivityof such catalyst systems in such processes. The discovery of the processof the instant invention, therefore, is believed to be a decided advancein the art.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process for the oxidationof nonaromatic hydrocarbons to produce maleic anhydride.

Another object of this invention is to provide a process for theproduction of maleic anhydride in excellent yields.

These and other objects, aspects, and advantages of this invention willbecome apparent to those skilled in the art from the accompanyingdescription and claims.

The above objects are achieved by the process disclosed herein for theproduction of maleic anhydride by the oxidation of a nonaromatichydrocarbon having at least four carbon atoms in a straight chain withmolecular oxygen or a molecular oxygen-containing gas in the vapor phaseat a temperature from about 300° C. to about 600° C. in the presence ofa phosphorus-vanadium mixed oxide catalyst, wherein the catalyst isprepared by:

(a) contacting a tetravalent vanadium compound, dissolved in an aqueous,non-oxidizing acid medium, with crystalline diphosphoric acid to form aphosphorus-vanadium mixed oxide catalyst precursor;

(b) crystallizing the catalyst precursor from the catalyst precursoraqueous solution in a controlled manner involving at least three cyclesof concentrating/diluting wherein a fraction of the liquid from about0.15 to about 0.85 is removed during the concentrating step and water isadded during the diluting step in an amount sufficient to provide awater added/solvent removed volume ratio from about 0.10 to about 10.0,with the proviso that the final liquid/initial liquid volume ratio isfrom about 0.20 to about 2.0;

(c) recovering the catalyst precursor crystals;

(d) drying the catalyst precursor;

(e) forming the dried catalyst precursor into structures; and

(f) calcining the catalyst precursor structures at a temperature fromabout 300° C. to about 600° C.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with this invention, a process is provided for theproduction of maleic anhydride by the oxidation of a nonaromatichydrocarbon having at least four carbon atoms in a straight chain withmolecular oxygen or a molecular oxygen-containing gas in the vapor phaseat a temperature from about 300° C. in the presence ofphosphorus-vanadium mixed oxide catalysts. Broadly described, thecatalysts employed in the instant process comprises contacting atetravalent vanadium compound, dissolved in an aqueous, non-oxidizingacid medium, with crystalline diphosphoric acid to form aphosphorus-vanadium mixed oxide catalyst precursor, crystallizing thecatalyst precursor, recovering the catalyst precursor, forming thecatalyst precursor into structures, and calcining the structuredcatalyst precursor to form the catalyst.

For purposes of this invention, the term "yield" means the ratio of themoles of maleic anhydride obtained to the moles of hydrocarbon feedstockintroduced into the reactor. The term "selectivity" means the ratio ofmoles of maleic anhydride obtained to the moles of hydrocarbon feedstockreacted or converted. The term "conversion" means the number of moles ofhydrocarbon feedstock reacted to the moles of hydrocarbon introducedinto the reactor. The term "gas hourly space velocity" or "GHSV" orsimply "space velocity" means the hourly volume of gaseous feedexpressed in cubic centimeters (cc) at room temperature (20° C.) andatmospheric pressure, divided by the catalyst bulk volume, expressed incubic centimeters, the term expressed as cc/cc/hour or hr⁻¹.

The tetravalent vanadium compounds suitable for use as a source ofvanadium in the catalysts employed in the process of the instantinvention are those tetravalent compounds known to the art. Thetetravalent vanadium can be obtained either by the use of a tetravalentvanadium compound or, alternatively, by the use of an easily accessiblepentavalent vanadium compound, such as vanadium pentoxide, which isreduced in situ to a tetravalent vanadium compound. Examples of suitabletetravalent vanadium compounds are vanadium tetrachloride (VCl₄);vanadium dioxide (VO₂), sometimes termed vanadium tetroxide (V₂ O₄);vanadium oxydibromide (VOBr₂); and vanadium oxydichloride (VOCl₂), whichis the preferred compound. In general, the tetravalent vanadiumcompounds suitable for use in the instant process are halides, oxides,and oxyhalides of vanadium. Examples of useful pentavalent vanadiumcompounds, which can be reduced in situ to obtain a tetravalent vanadiumcompound are vanadium pentoxide (V₂ O₅), which is the preferredcompound; vanadium oxytribromide (VOBr₃); vanadium oxytrichloride(VOCl₃); and vanadium pentachloride (VCl₅).

Crystalline diphosphoric acid, also known as pyrophosphoric acid, H₄ P₂O₇, is employed as the source of (pentavalent) phosphorus in thecatalysts employed in the instant process. Crystalline (or solid)diphosphoric acid is obtainable by procedures well known to thoseskilled in the art. In general, crystalline diphosphoric acid isobtained by the spontaneous crystallization of a polyphosphoric acidmixture containing 79.8±0.2% phosphorus pentoxide (P₂ O₅). Such a liquidpolyphosphoric acid may be prepared by (1) dehydration of 85%orthophosphoric acid, (2) dissolving P₂ O₅ in 85% orthophosphoric acid,(3) adding water to commercial "tetraphosphoric acid," or (4) allowingPOCl₃ to react with 85% orthophosphoric acid. For additional informationregarding the preparation of crystalline diphosphoric acid, see Malowan,Inorganic Synthesis. Audrieth, Ed., Vol. III, pp. 96-98, McGraw Hill,New York, N.Y. 1950, which is herein incorporated by reference.

To prepare precursors to the catalysts employed in the instant process,a suitable vanadium compound dissolved in an aqueous, non-oxidizing acidmedium is contacted with crystalline diphosphoric acid and heated toinduce complete solution and form the desired catalyst precursor.

When a pentavalent vanadium compound, such as V₂ O₅, is employed as thestarting material, it must be reduced to the tetravalent valence state.The desired reduction is readily accomplished by contacting thepentavalent vanadium compound with a suitable reducing agent in an acidmedium. As is well known to those skilled in the art, hydrohalic acidsor oxalic acid solutions, which are mild reducing agents, can serve notonly as the acid but also as the reducing agent for the pentavalanetvanadium. Among these compounds, hydrochloric acid is preferred.

The atom ratio of phosphorus to vanadium in the starting material isimportant since it controls, in part, the phosphorus/vanadium (P/V) atomratio in the final catalyst. The catalysts employed in the process ofthe instant invention exhibit a P/V atom ratio from about 0.5 to about2.0, with a P/V atom ratio of about 0.95 to about 1.2 being preferred.As previously noted, the atom ratio in the catalyst is determined, inpart, by the P/V atom ratio in the starting material as charged to thereactor. However, since the catalyst precursor is normally recovered byfiltration or centrifugation and decantation, the analyzed P/V atomratio is usually slightly less than the corresponding charged ratio.Typically, a P/V (charge) atom ratio of about 1.0 yields a catalysthaving a phosphorus/vanadium (analyzed) atom ratio of about 0.95, whilea charged atom ratio of about 1.2 yields a catalyst precursor (andcatalyst) having an analyzed atom ratio of about 1.0, thus indicatingthat a portion of the charged phosphorus relative to the chargedvanadium is lost during the recovery step.

After the phosphorus-vanadium mixed oxide catalyst precursors have beenformed by heating the tetravalent vanadium compound and the diphosphoricacid, it is necessary to recover the catalyst precursors. Numerous priorart techniques for recovering catalyst precursors from solution are wellknown to those skilled in the art. However, such prior arttechniques--evaporating to dryness and rapid or uncontrolledprecipitation or crystallization, for example--are unsatisfactory foruse in preparing the catalysts employed in the instant process in thatthe resulting catalysts exhibit none of the advantages characteristic ofthe catalysts prepared as desribed herein. Thus, the catalyst precursoris crystallized from the aqueous solution by a controlledcrystallization technique. The technique involves at least three cyclesof concentrating/diluting (or distillation/water addition) of thecatalyst precursor aqueous solution (mixture). In each cycle, a fractionof the solvent (liquid) ranging from 0.15 to about 0.85 is removedduring the concentrating step and water is added in an amount sufficientto provide a water added/solvent removed volume ratio from about 0.10 toabout 10.0. The final liquid/initial liquid (after formation of thecatalyst precursor) volume ratio, however, should range from about 0.20to about 2.0. Within this range, the amount of liquid present issufficiently small to minimize the retention of catalyst precursorcrystals in solution while at the same time sufficiently large tofacilitate ease of separating the catalyst precursor crystals from thesupernatant liquid. Separation or recovery of the catalyst precursorcrystals is readily accomplished by filtration, centrifugation anddecantation, and the like.

The maximum number of concentration/diluting cycles is not narrowlycritical. All that is necessary is that at least three of such cyclesare performed and that crystallization has occurred to an extentsufficient to provide a substantial amount of crystalline catalystprecursor. In general, however, little, if any, advantage is observedbeyond a maximum of six cycles.

The theory of the controlled crystallization is not completelyunderstood. However, it is believed that the concentration/dilutioncycles cause repetitive crystallizations/dissolutions of the catalystprecursor. This, in turn, results in the formation of crystals which aresmaller and more uniform than crystals obtained from prior artprocedures, such as evaporation to dryness.

After the catalyst precursors have been recovered from the solution,they are then formed into structures suitable for use in a maleicanhydride reactor. Techniques for forming appropriate structures fromthe catalyst precursors for use in a fluidized bed reactor or in a fixedbed, heat exchanger type reactor are well known to those skilled in theart. For example, the catalyst precursors can be comminuted for use in afluidized bed reactor. Similarly, the catalyst precursors can bestructured for use in a fixed bed, heat exchanger type reactor byprilling or tableting the precursors.

After the catalyst precursors have been formed into the desiredstructures, they can be calcined in a molecular oxygen-containingatmosphere, such as air, at temperatures from about 300° C. to about600° C. for a suitable period of time, usually at least two hours, toconvert the catalyst precursors to the active catalyst.

The exact calcination conditions employed are not narrowly critical. Allthat is necessary is that the catalyst precursors be calcined untilabout 15 atom percent to about 90 atom percent of the vanadium has beenconverted to pentavalent vanadium. If more than about 90 atom percent ofthe vanadium is oxidized to pentavalent vanadium, usually caused bycalcining too long, or at too high a temperature, the selectivity of theresultant catalyst and the yield of maleic anhydride decrease markedly.On the other hand, oxidation of less than about 15 atom percent of thevanadium during the calcination does not seem to provide a catalysthaving sufficient activity to convert suitable nonaromatic hydrocarbonfeedstocks to maleic anhydride at a reasonable rate. In general,calcination at temperatures from about 400° C. to about 500° C. for aperiod of time from about four hours to about eight hours, usually sixhours, is sufficient.

The phosphorus-vanadium mixed oxide catalysts can be used (in a suitablereactor) to convert nonaromatic hydrocarbons to maleic anhydride. Amixture of hydrocarbon and a molecular oxygen-containing gas, (includingmolecular oxygen), such as air, can be contacted with the catalyst attemperatures between about 300° C. and 600° C. at concentrations of fromabout one mole percent to about 10 mole percent hydrocarbon at a gashourly space velocity (GHSV), or simply space velocity, up to about4,000 hr⁻¹ to produce maleic anhydride. However, the initial yield ofmaleic anhydride may be low; and if this is the case, the catalyst, aswill occur to those skilled in the art, can be "conditioned" bycontacting the catalyst with low concentrations of hydrocarbon andmolecular oxygen-containing gas at low space velocities for a period oftime before production operations begin.

The catalysts employed in process of the instant invention are useful ina variety of reactors to convert nonaromatic hydrocarbons to maleicanhydride. The catalyst may be used in a fixed bed reactor usingtablets, pellets, or the like, or in a fluidized bed reactor usingcatalysts preferably having a particle size of less than about 300microns. Details of the operation of such reactors are well known tothose skilled in the art.

The reaction to convert nonaromatic hydrocarbons to maleic anhydriderequires only contacting the hydrocarbon admixed with a molecularoxygen-containing gas, such as air or molecular oxygen enriched air,with the catalyst at elevated temperatures. In addition to thehydrocarbon and molecular oxygen, other gases, such as nitrogen orsteam, may be present or added to the reactant feedstream. Typically,the hydrocarbon is admixed with the molecular oxygen-containing gas,preferably air, at a concentration of about one mole percent to about 10mole percent hydrocarbon and contacted with the catalyst at a spacevelocity of about 100 hr⁻¹ to about 4,000 hr⁻¹ at a (bath) temperaturefrom about 300° C. to about 600° C., preferably about 1450 hr⁻¹ andabout 325° C. to about 425° C., to provide an excellent selectivity toand yield of maleic anhydride.

The catalyst prepared in accordance with the instant process areparticularly useful in fixed bed (tube), heat exchanger type reactors.The tubes of such reactors can vary in diameter from about 0.635 cm(0.25 inch) to about 3.81 cm (1.5 inches) and the length can vary fromabout 15.24 cm (6 inches) to about 304.8 cm (10 feet) or more. It isdesirable to have the surfaces of the reactors at relatively constanttemperatures, and some medium to conduct heat from the reactors isnecessary to aid temperature control. Non-limiting examples of suchmedia include Woods metal, molten sulfur, mercury, molten lead, andeutectic salt baths. A metal block reactor whereby the metal surroundingthe tube acts as a temperature regulating body can also be used. Thereactor or reactors can be constructed or iron, stainless steel, carbonsteel, glass, and the like.

Pressure is not critical in the reaction to convert nonaromatichydrocarbons to maleic anhydride. The reaction may be conducted atatmospheric, superatmospheric, or subatmospheric pressures. It willgenerally be preferred, however, for practical reasons to conduct thereaction at or near atmospheric pressure. Generally, pressures fromabout 1.013×10² kPa-G (14.7 psig, 1 atm.) to about 1.38×10² kPa-G (20.0psig) may be conveniently employed.

Maleic anhydride produced by using the process of the instant inventioncan be recovered by any means well known to those skilled in the art.For example, maleic anhdride can be recovered by direct condensation orby absorption in suitable media with subsequent separation andpurification of the anhydride.

A large number of nonaromatic hydrocarbons having from four to 10 carbonatoms can be converted to maleic anhydride by using the catalystprepared according to the instant process. It is only necessary that thehydrocarbon contain not less than four carbon atoms in a straight chain.As an example, the saturated hydrocarbon n-butane is satisfactory, butisobutane (2-methylpropane) is not satisfactory for conversion to maleicanhydride although its presence is not harmful. In addition to n-butane,other suitable saturated hydrocarbons include the pentanes, the hexanes,the heptanes, the octanes, the nonanes, the decanes, and mixtures of anyof these, with or without n-butane, so long as an unbranched chainhaving at least four carbon atoms are present in the saturatedhydrocarbon molecule.

Unsaturated hydrocarbons are also suitable for conversion to maleicanhydride using the catalyst prepared according to the instant process.Suitable unsaturated hydrocarbons include the butenes (1-butene and2-butene), 1,3-butadiene, the pentenes, the hexenes, the heptenes, theoctenes, the nonenes, the decenes, and mixtures of any of these, with orwithout the butenes, again, so long as the requisite unbranched C₄hydrocarbon chain is present in the molecule.

Cyclic compounds, such as cyclopentane, cyclopentene, cyclohexane, orcyclohexene, are also satisfactory feed materials for conversion tomaleic anhydride.

Of the aforementioned feedstocks, n-butane is the preferred saturatedhydrocarbon and the butenes are the preferred unsaturated hydrocarbons,with n-butane being most preferred of all feedstocks.

It will be noted that the aforementioned feedstocks need not necessarilybe pure substances, but can be technical grade hydrocarbon.

The principal product from the oxidation of the above-noted feedmaterials is maleic anhydride, although small amounts of citraconicanhydride (methylmaleic anhydride) may also be produced when thefeedstock is a hydrocarbon containing more than four carbon atoms in astraight chain.

The following specific examples illustrating the best currently-knownmethod of practicing this invention are described in detail in order tofacilitate a clear understanding of the invention. It should beunderstood, however, that the detailed expositions of the application ofthe invention while indicating preferred embodiments, are given by wayof illustration only and are not to be construed as limiting theinvention since various changes and modification within the spirit ofthe invention will become apparent to those skilled in the art from thisdetailed description.

EXAMPLE 1

(a) Diphosphoric Acid

To a three-liter, round bottom flask equipped with a paddle stirrer wascharged 276.6 g (2.84 moles) of 100.54% phosphoric acid (H₃ PO₄) with97.6 g (0.64 mole) of phosphorus oxytrichloride (POCl₃). The mixture wasstirred and heated to 73° C. for three hours, during which period largeamounts of hydrogen chloride (HCl) gas were evolved. Following thisperiod, the reaction mass was heated to 105° C. for one hour and cooledto room temperature. The resulting liquid was allowed to stand at roomtemperature for four days, after which period, the liquid crystallizedto form solid diphosphoric acid, H₄ P₂ O₇.

(b) Catalyst

A five-liter, round bottom flask fitted with a paddle stirrer,thermometer, heating mantle, and reflux condenser with water scrubberwas charged with 3436 ml (4088.8 g, 41.45 moles) of concentrated (37%)hydrochloric acid and 295.0 g (1.62 moles) of high-purity vanadiumpentoxide (V₂ O₅). The mixture was heated to 103° C., at whichtemperature reflux was attained. When chlorine (Cl₂) gas was no longerevolved from the flask, a total of 2600 ml of solvent was removed fromthe reaction by distillation. To this concentrated VOCl₂ solution wasadded a solution of 303.1 g (1.78 moles) of H₄ P₂ O₇ [the entire massfrom Section (a) above] in 590 ml of H₂ O (P/V atom mole of 1.05). Thisblue solution was held at 90° C. overnight. Thereafter a series ofconcentration/dilution (distillation/water addition) cycles wasperformed as follows:

    ______________________________________                                        Sol'n     Solv. Dist. H.sub.2 O Added,                                                                         H.sub.2 O Added/-                            Cycle ml      ml     Fract. ml       Solv. Dist.                              ______________________________________                                        1     1426    600    0.42   300      0.50                                     2     1126    600    0.53   300      0.50                                     3      826    325    0.39   300      0.92                                     4      801    400    0.50   1000     2.50                                     ______________________________________                                    

After cycle 2, a small quantity of crystals was observed, which amountvisibly increased after the distillation step of cycle 4. The reactionmixture, which was quite thick at this point, was treated with one literof water (final liquid/initial liquid volume ratio of 0.98), cooled to70° C., and suction filtered. The filter cake was thoroughly washed withwater and dried for 72 hours at 130° C. The granulated [-24 mesh (U.S.Standard Sieve Size)] powder was light blue in color (P/V atom ratio of1.00). The dried, granulated precursor powder was mixed with one weightpercent of powdered graphite and formed into 0.48-cm (0.1875-inch)cylinders (tablets) having an average (side) crush strength of 8.90newtons (N, 2.00 lbs.). The tablets were calcined in air at 400° C. forsix hours. The calcined catalyst was performance tested in a 2.54-cm(1-inch) inside diameter×15.24-cm (6-inch) long tubular fixed-bedreactor at 1450 hr⁻¹ and 2600 hr⁻¹ space velocities and 1.5 mole percentn-butane-in-air. The parameters and results are tabulated in Tables 1and 2.

EXAMPLE 2

(a) Diphosphoric Acid

To a two-liter, round bottom flask, equipped with a stainless steelpaddle stirrer, was added 1000.0 g (11.17 moles) of 116% phosphoric acid(H₃ PO₄) and 53.3 g (2.96 moles) of H₂ O at 25° C. The mixture wasstirred while its temperature rose to 90° C. The solution (110.12% H₃PO₄) was cooled to 35° C. and 41.5 g (0.23 mole) of seed diphosphoricacid (H₄ P₂ O₇) crystals were added to the H₃ PO₄. Stirring wascontinued for six hours until it was impossible to continue because ofcrystallization of the H₄ P₂ O₇. The semi-solid mass was kept at 22° C.overnight (approximately 16 hours). The resulting solid mass was chippedout of the flask and stored in a desiccator over Drierie® (anhydrouscalcium sulfate).

(b) Catalyst

The procedure described in Example 1 was repeated except thatcrystalline H₄ P₂ O₇ from Section (a) above was employed.

To a five-liter, round bottom flask fitted with a reflux condenser,paddle stirrer, exit gas water scrubber, thermometer and heating mantlewas charged 3436 ml (4088.8 g, 41.45 moles) of concentrated (37%)hydrochloric acid and 295.0 g (1.62 moles) of high-purity V₂ O₅. Themixture was heated to reflux and held at reflux for 18 hours. After thistime, 2600 ml of distillate was removed at atmospheric pressure and asolution of 291.57 g (1.64 moles) of H₄ P₂ O₇ in 600 ml of water wasadded to the blue vanadium solution (P/V atom ratio of 1.01). A seriesof distillation/water addition cycles was then performed as follows:

    ______________________________________                                        Sol'n     Solv. Dist. H.sub.2 O Added,                                                                          H.sub.2 O Added/-                           Cycle ml      ml      Fract.                                                                              ml        Solv. Dist.                             ______________________________________                                        1     1436    600     0.42  300       0.52                                    2     1136    350     0.31  300       0.86                                    3     1086    425     0.39  300       0.71                                    4      961    325     0.34  300       0.92                                    5      936    300     0.32  1300      4.33                                    6     1936    1400    0.72  150       0.11                                    ______________________________________                                    

After these six cycles, the reaction mixture was a thick blue slurry,with a final liquid/initial liquid volume ratio of 0.48. The mixture wascooled to room temperature and suction filtered. The filter cake wasthoroughly washed with water until the filtrate was clear, driedovernight at 130° C., and granulated to give a blue granular powder. Thedried precursor powder was mixed with one weight percent of powderedgraphite and formed into 0.48-cm (0.1875-inch) cylinders (tablets). Thetablets were calcined in air at 400° C. for six hours to yield thecatalyst having a P/V atom ratio of 0.99. The catalyst was performancetested as described in Example 1. The parameters and results aretabulated in Tables 1 and 2.

EXAMPLE 3

A twelve-liter, round bottom flask, fitted with paddle stirrer,thermometer, two-zone heating mantle and reflux condenser (attached to awater scrubbing column), was charged with 6872 ml (8177.7 g, 82.90moles) of concentrated (37%) hydrochloric acid and 590.0 g (3.24 moles)of high-purity V₂ O₅. The mixture was heated from approximately 35° C.to reflux temperature (102°-104° C.) Reflux was maintained for 20 hours.During the heating/reflux time, the orange mixture first turned to adark red solution, then gradually to a dark blue solution. After thehold period, a total of 5200 ml of solvent was distilled from thereaction solution. The solution was cooled to 50° C. and a mixture of583.14 g (3.28 moles) of crystalline H₄ P₂ O₇ (from seededcrystallization of 110.12% H₃ PO₄) in 1200 ml of water was added. Thereaction mixture was heated to reflux temperature. Thereafter, a seriesof distillation/water addition cycles was performed as follows:

    ______________________________________                                        Sol'n     Solv. Dist. H.sub.2 O Added,                                                                          H.sub.2 O Added/-                           Cycle ml      ml      Fract.                                                                              ml        Solv. Dist.                             ______________________________________                                        1     2872    1200    0.42  600       0.50                                    2     2272    700     0.31  600       0.86                                    3     2172    850     0.39  600       0.71                                    4     1922    700     0.36  600       0.86                                    5     1822    800     0.44  1000      1.25                                    6     2022    1650    0.82  500       0.30                                    ______________________________________                                    

A total of 11,972 ml of liquid was added to the flask. A total of 11,100ml of liquid was distilled out to give a final liquid/initial liquidvolume ratio of 0.30.

Crystallization of a blue product was observed after the distillationstep of cycle 3. The amount of precipitate increased throughout therepetitive cycles. After the final cycle (cycle 6), the blue slurry wascooled to room temperature and suction filtered. The filter cake wasthoroughly washed with water, dried at 130° C. for 60 hours andgranulated [-25 mesh (U.S. Standard Sieve Size)] to yield a blue powderhaving a P/V mole ratio of 1.01. The dried catalyst precursor powder wasmixed with one weight percent of powdered graphite and formed into0.48-cm (0.1875-inch) cylinders (tablets) having an average (side) crushstrength of 22.25 N (5.00 lbs). The tablets were calcined in air at 400°C. for six hours to yield the catalyst having a P/V atom ratio of 1.00.The catalyst was performance tested as described in Example 1 exceptthat a 2.54-cm (1 inch) inside diameter×121.91-cm (4 foot) tubularfixed-bed reactor was employed. The parameters and results are tabulatedin Tables 1 and 2.

EXAMPLE 4

A three-liter, round bottom flask fitted with a paddle stirrer,thermometer, addition funnel, reflux condenser, water scrubber, andheating mantle was charged with 2000 ml (2380.0 g, 24.13 moles) ofconcentrated (37%) aqueous HCl, 100 ml of isobutyl alcohol, and 162.2 g(0.89 mole) of high-purity V₂ O₅. The mixture was heated, with stirring,to reflux and maintained at reflux for one hour. The solution was thendistilled at atmospheric pressure to remove 1400 ml of distillate. Tothe residual solution was added 160.35 g (0.90 mole) of crystalline H₄P₂ O₇ (from seeded crystallization of 110.12% H₄ PO₄) dissolved in 100ml of concentrated (37%) aqueous HCl. An additional 50 ml ofconcentrated aqueous HCl was used to rinse residual H₄ P₂ O₇ into thereaction flask. A series of distillation/water addition cycles was thenperformed as follows:

    ______________________________________                                        Sol'n     Solv. Dist. H.sub.2 O Added,                                                                         H.sub.2 O Added/-                            Cycle ml      ml     Fract. ml       Solv. Dist.                              ______________________________________                                        1     850     350    0.41   100      0.29                                     2     600     125    0.21   100      0.80                                     3     575     100    0.17   100      1.00                                     4     575     100    0.17   200      2.00                                     5     675     150    0.22   1000     6.67                                     ______________________________________                                    

After cycle 3, a small quantity of crystals was observed, which amountvisibly increased after cycle 4. After cycle 5, the mixture, having afinal liquid/initial liquid volume ratio of 1.79, was cooled to 70° C.and suction filtered. The filter cake was washed with 200 ml ofdeionized water, filtered, dried overnight in air at 130° C., andgranulated [-25 mesh (U.S. Standard Sieve Size)] to yield a bluegranular powder. The dry powder was formed into tablets and calcined asdescribed in Example 3. The calcined tablets were performance tested at1450 hr⁻¹ as described in Example 1. The parameters and results aretabulated in Tables 1 and 2.

EXAMPLES 5-8 (Comparative)

Examples 5-8 illustrate the criticality of the combination ofcrystalline diphosphoric acid and the controlled crystallizationprocedure to produce superior catalysts.

    ______________________________________                                                 Critical Feature Present                                             Example    H.sub.4 P.sub.2 O.sub.7                                                                Controlled Crystallization                                ______________________________________                                        5          No       Yes                                                       6          Yes      No                                                        7          Yes      No                                                        8          No       Yes                                                       ______________________________________                                    

EXAMPLE 5 (Comparative)

A three-liter, round bottom flask fitted as described in Example 4 wascharged with 2000 ml (2380.0 g, 24.13 moles) of concentrated (37%)aqueous HCl, 100 ml of isobutyl alcohol, and 162.2 g (0.89 mole) ofhigh-purity V₂ O₅. The mixture was heated, with stirring, to reflux andmaintained at reflux for one hour. The solution was then distilled atatmospheric pressure to remove 1400 ml of distillate. To the residualsolution was added 122 ml (206.5 g, 1.80 moles) of 85.5% H₃ PO₄.Thereafter, a series of distillation/water addition cycles was performedto give a final liquid/initial liquid volume ratio of 1.92.

    ______________________________________                                        Sol'n     Solv. Dist. H.sub.2 O Added,                                                                          H.sub.2 O Added/-                           Cycle ml      ml      Fract.                                                                              ml        Solv. Dist.                             ______________________________________                                        1     822     155     0.18  100       0.65                                    2     767     160     0.21  100       0.63                                    3     707     125     0.18  100       0.80                                    4     682     125     0.18  200       1.60                                    5     757     200     0.26  200       1.00                                    6     757     200     0.26  500       2.50                                    7     1057    480     0.45  500       1.04                                    8     1077    500     0.46  500       1.00                                    9     1077    500     0.46  1000      2.00                                    ______________________________________                                    

A small amount of crystals appeared during cycle 4 which visiblyincreased following each cycle through cycle 8. After cycle 9, themixture was cooled to 70° C. and suction filtered. The filter cake wasthoroughly washed with water, dried overnight in air at 130° C., andgranulated [-18 mesh (U.S. Standard Sieve Size)] to yield a bluegranular powder. The dry powder was spheroidized on a 40.64-cm (16-inch)pan pelletizer with water spray. The spheres were dried overnight(approximately 16 hours) at 130° C. and screened to yield spheres havinga diameter from about 0.48 cm to about 0.67 cm. The dry spheres werecalcined in air at 400° C. for six hours. The calcined spheres wereperformance tested at 1450 hr⁻¹ as described in Example 1. Theparameters and results are tabulated in Tables 1 and 2.

EXAMPLE 6 (Comparative)

A twelve-liter, round bottom flask, fitted as described in Example 3 wascharged with 7570 ml (9008.3 g, 91.32 moles) of concentrated (37%)aqueous HCl and 644.0 g (3.54 moles) of high-purity V₂ O₅. The mixturewas heated to reflux and refluxed for about 18 hours. After the refluxperiod, a total of 6100 ml of solvent was distilled from the reactionsolution. The residual blue syrup was cooled to room temperature andquantitatively transferred to a five-liter beaker with 1500 ml of water.To the resulting solution was added 645.98 g (3.63 moles) ofcrystal-line H₄ P₂ O₇ (from seeded crystallization) of 110.12% H₃ PO₄)at a rate sufficient to maintain the temperature below 45° C. (P/V atomratio of 1.025). The blue solution was heated at 70° C. overnight(approximately 16 hours) and concentrated at its boiling point to athick slurry (near dryness). The slurry was taken up in 1000 ml of waterand filtered. The filter cake was washed with water until the filtratewas clear, dried at 130° C., and granulated [-25 mesh (U.S. StandardSieve Size)] to yield a blue granular powder. The dry powder was mixedwith 1.5 weight percent of powdered graphite and formed into 0.48-cm(0.1875-inch) cylinders (tablets). The tablets were tray calcined in airat 400° C. for six hours to yield the catalyst having a P/V atom ratioof 1.00. The catalyst was performance tested as described in Example 1.The parameters and results are tabulated in Tables 1 and 2.

EXAMPLE 7 (Comparative)

A three-liter, round bottom flask fitted with a paddle stirrer,thermometer addition funnel, reflux condenser, water scrubber, andheating mantle was charged with 1750 ml (2082.5 g, 21.11 moles) of 12 N(37%) aqueous HCl and 1622.5 g (8.91 moles) of V₂ O₅. The mixture wasstirred at a moderate rate to maintain the solids in suspension andheated to reflux. The reflux temperature was maintained for 20 hours toyield a blue solution. The solution was distilled to remove 13,125 ml ofdistillate and treated with 1667.1 g (9.37 moles) of H₄ P₂ O₇ (fromseeded crystallization of 110.12% H₃ PO₄) and 324.0 g of H₂ O(equivalent to 85% H₃ PO₄. The mixture was distilled to remove anadditional 2000 ml of distillate. The residual slurry was dried at 130°C. to yield a blue product [-24 mesh (U.S. Standard Sieve Size)]. Thedry powder was spheroidized on a 40.64-cm (16-inch) pan pelletizer withwater spray. The spheres were dried overnight (approximately 16 hours)at 130° C. and screened to yield spheres having a diameter from about0.48 cm to about 0.67 cm. The dry spheres were calcined in air at 400°for six hours. The calcined spheres were performance tested as describedin Example 3. The parameters and results are tabulated in Tables 1 and2.

EXAMPLE 8 (Comparative)

A three-liter, round bottom flask fitted as described in Example 3 wascharged with 1743 ml (2074.4 g, 21.03 moles) of concentrated (37%)aqueous HCl and 162.2 g (0.89 mole) of high-purity V₂ O₅. The mixturewas heated to reflux and maintained at reflux temperature, withstirring, for two hours. The resulting solution was distilled to remove1410 ml of colorless distillate. The remaining solution was reacted with122 ml (206.5 g, 1.80 moles) of 85.5% H₃ PO₄ and 200 ml of water.Thereafter, a series of distillation/water addition cycles was performedas follows:

    ______________________________________                                        Sol'n     Solv. (H.sub.2 O/HCl)                                                                      H.sub.2 O Added,                                                                         H.sub.2 O Added/-                           Cycle ml      ml      Fract. ml       Solv. Dist.                             ______________________________________                                        1     655     205     0.31   200      0.98                                    2     650     250     0.38   200      0.80                                    3     600     100     0.17   200      2.00                                    ______________________________________                                    

After cycle 3, the mixture, having a final liquid/initial liquid volumeratio was 1.07, was cooled to 70° C. and suction filtered. The filtercake was resuspended in 1000 ml of water, filtered, and dried overnight(approximately 16 hours) at 130° C. to yield a blue powder [-24 mesh(U.S. Standard Sieve Size)] having a P/V atom ratio of 0.99. The drypowder was mixed with one weight percent of powdered graphite and formedinto 0.48-cm (0.1875-inch) cylinders having an average crush strength of22.25 N (5.00 lbs). The tablets were calcined in air at 400° C. for sixhours. The calcined tablets were performance tested at 1450 hr⁻¹ asdescribed in Example 1. The parameters and results are tabulated inTables 1 and 2.

EXAMPLE 9 (Comparative)

This Example illustrates a typical prior art catalyst prepared from 85percent orthophosphoric acid and a concentrating/water dilutiontechnique. The catalyst was prepared according to the proceduredescribed in Example 1 of U.S. Pat. No. 4,085,122.

To a five-liter, round bottom flask fitted with a mechanical stirrer,condenser, and an HCl gas scrubber was charged 400.0 g (2.20 moles) ofhigh-purity V₂ O₅ and 2712 ml (3200.0 g, 32.44 moles) of concentrated(37%) aqueous HCl. The suspension was heated to 100°±2° C. in 80±20minutes, and was held at this temperature for two hours, during whichperiod chlorine (Cl₂) gas was evolved. The resulting deep blue solutionwas cooled slightly and 39.19 g (0.31 mole) of oxalic acid dihydrate[(COOH₂.2H₂ O], 325 ml (548.0 g, 4.75 moles) of 85% H₃ PO₄, and 280 mlof deionized water were added slowly (P/V atom ratio of 1.08). The flaskwas set up for distillation and two cycles of distillation/wateraddition were performed to give a final liquid/initial liquid volumeratio of 0.42.

    ______________________________________                                        Sol'n     Solv. Dist.            H.sub.2 O Added/-                            Cycle  ml     ml      Fract. H.sub.2 O Added                                                                       Solv. Dist.                              ______________________________________                                        1      3317   2418    0.73   500     0.21                                     2      1399    800    0.57   800     1.00                                     ______________________________________                                    

The resulting precipitate was collected by filtration, washed with four250-ml portions of deionized H₂ O, and air dried. The dried material wassuspended in 1000 ml of deionized water, boiled at 100° C. for twohours, filtered, washed, air dried, and heated overnight (approximately16 hours) at 120° C. in a forced draft oven. The granulated powder wasmixed with one weight percent of powdered graphite and formed into0.48-cm (0.1875-inch) tablets and calcined in air at 400° C. for sixhours. The catalyst was performance tested as described in Example 1.The parameters and results are tabulated in Tables 1 and 2.

EXAMPLE 10 (Comparative)

This Example illustrates an early prior art catalyst prepared in anaqueous medium using hydrochloric acid as the reducing agent. Thecatalyst was prepared according to the procedure described in Example 2of U.S. Pat. No. 3,293,268.

A three-liter, round bottom flask fitted with a paddle stirrer,thermometer, addition funnel, reflux condenser, water scrubber, andheating mantle was charged with 1750 ml (2082.5 ml, 21.11 moles) of 12 Nhydrochloric acid (HCl) and 134.4 g (0.74 mole) of V₂ O₅. The mixturewas stirred at a moderate rate to maintain the solids in suspension andheated to 90° C. over a two-hour period to give a blue solution. To thissolution was added, over a 20-minute period, 105 ml (177.6 g, 1.55moles) of 85.5% phosphoric acid (P/V atom ratio of 1.05). During thephosphoric acid addition, the reaction temperature did not exceed 91° C.The reaction mixture was cooled to 70° C. and transferred to a porcelaindish and dried in an oven at 130° C. for about 50 hours. The drymaterial was then heated in air at 365° C. for two hours. The tabletswere calcined in dry air for six hours at 400° C. The catalyst wasperformance tested as described in Example 1. The parameters and resultsare tabulated in Tables 1 and 2.

                                      TABLE 1                                     __________________________________________________________________________                                                MEAN   TOTAL BET                                       P/V       AVERAGE POROS-                                                                             PORE   PORE  SURFACE                    EMPIRICAL                                                                             FORM   ATOM      VANADIUM                                                                              ITY  DIAMETER                                                                             VOLUME                                                                              AREA                 EX.                                                                              NO.                                                                              FORMULA.sup.1                                                                         (SIZE, cm)                                                                           RATIO                                                                              V.sup.+4 /V.sup.t.spsp.2                                                           VALENCE %    μm  cc/g  m.sup.2 /g           __________________________________________________________________________     1 1  P.sub.1.0 V.sub.1.0 O.sub.x                                                           Tablets (0.48)                                                                       1.00 19.93                                                                              4.80    35.20                                                                              0.20   0.22  17.00                 2 2  P.sub.0.99 V.sub.1.0 O.sub.x                                                          "      0.99 17.21                                                                              4.83    38.65                                                                              0.11   0.16  8.30                  3 3  P.sub.1.0 V.sub.1.0 O.sub.x                                                           "      1.00 18.44                                                                              4.82    32.45                                                                              0.15   0.17  8.50                  4.sup.3                                                                         4  P.sub.1.03 V.sub.1.0 O.sub.x                                                          "      1.03 27.46                                                                              4.73    35.98                                                                              0.18   0.21  --                    5.sup.3,4                                                                       5  P.sub.1.02 V.sub.1.0 O.sub.x                                                          Spheres                                                                              1.02 24.67                                                                              4.75    27.94                                                                              0.13   0.15  --                                 (0.48-0.67)                                                      6.sup.4                                                                         6  P.sub.1.04 V.sub.1.0 O.sub.x                                                          Tablets (0.48)                                                                       1.04.sup.5                                                                         26.42                                                                              4.74    33.71                                                                              0.16   0.18  5.90                  7.sup. 4                                                                        7  P.sub.1.05 V.sub.1.0 O.sub.x                                                          Spheres                                                                              1.05 30.16                                                                              4.70    41.37                                                                              0.19   0.26  3.50                               (0.48-0.67)                                                      8.sup.4                                                                         8  P.sub.0.99 V.sub.1.0 O.sub.x                                                          Tablets (0.48)                                                                       0.99 26.51                                                                              4.74    --   --     --    --                    9.sup.4                                                                         9  P.sub.1.04 V.sub.1.0 O.sub.x                                                          "      1.04.sup.6                                                                         30.20                                                                              4.70    53.20                                                                              0.16   0.29  --                   10.sup.4                                                                         10 P.sub.1.05 V.sub.1.0 O.sub.x                                                          "      1.05 28.10                                                                              4.72    31.30                                                                              0.12   0.18  4.20                 __________________________________________________________________________     .sup.1 Subscript x is a number taken to satisfy the valence requirements      of phosphorus and vanadium.                                                   .sup.2 V.sup.t represents total vanadium.                                     .sup.3 The aqueous medium contained added isobutyl alcohol.                   .sup.4 Comparative example.                                                   .sup.5 Apparently due to a greater relative loss of vanadium than             phophorus during the recovery of the crystalline catalyst precursor since     the charged P/V atom ratio was 1.025.                                         .sup.6 The analyzed P/V atom ratio for the filtered catalyst was 1.04         rather than 1.08 as claimed in Example 1 of U.S. Pat. No. 4,085,122.     

                                      TABLE 2                                     __________________________________________________________________________           CATALYST REACTOR                                                                           SPACE                                                     CATALYST                                                                             CHARGE DENSITY                                                                             VELOCITY                                                                              n-BUTANE                                                                             TEMPERATURE, °C.                                                                   CONV.                                                                              SEL.  YIELD               NO.    (kg/m.sup.3) × 10.sup.3                                                              hr.sup.-1                                                                             mole % BATH REACTION                                                                             mole %                                                                             mole                                                                                mole                __________________________________________________________________________                                                              %                   1      0.94         1450    1.5    382  408    80.4 70.3  56.5                "      "            2600    1.5    397  436    72.1 65.8  47.4                2      0.96         1450    1.5    387  414    80.0 70.2  56.1                "      "            2600    1.5    399  439    71.5 66.1  47.2                3      1.17         1450    1.5    390  436    80.1 71.3  57.1                "      "            2600    1.5    413  460    70.6 67.6  47.7                4      0.96         1450    1.5    388  419    79.3 69.8  55.4                "      "            2600    1.5    404  455    71.2 64.4  45.9                .sup. 5.sup.1                                                                        0.98         1450    1.5    402  438    78.1 65.5  50.7                .sup. 6.sup.1                                                                        1.02         1450    1.5    402  423    80.0 67.0  53.5                "      "            2600    1.5    414  450    71.7 61.1  43.8                .sup. 7.sup.1                                                                        0.77         1450    1.5    432  470    80.4 62.0  49.8                .sup. 8.sup.1                                                                        0.98         1450    1.5    399  433    79.5 65.5  52.0                "      "            2600    1.5    416  463    71.2 60.3  42.9                .sup. 9.sup.1                                                                        0.93         1450    1.5    390  424    79.1 67.6  53.5                "      "            2600    2.0    398  483    64.4 60.3  38.8                10.sup.1                                                                             0.92         1450    1.5    407  431    79.0 62.1  49.1                __________________________________________________________________________     .sup.1 Comparative catalyst.                                             

Comparison of the yields of maleic anhydride obtained with catalysts 1-4with those obtained with comparative catalysts 5-10 clearly demonstratesthe advantages of the instant process to prepare catalysts in aqueousmedia in that the yields are significantly higher for catalysts 1-4 whencompared with comparative catalysts 5-10.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a process that fully satisfies the objects andadvantages set forth hereinabove. While the invention has been describedwith respect to various specific examples and embodiments thereto andthat many alternatives, modifications, and variations will be apparentto those skilled in the art in light of the foregoing description.Accordingly, it is intended to embrace all such alternatives,modifications, and variations as fall within the spirit and broad scopeof the invention.

What is claimed is:
 1. A process for the production of maleic anhydrideby the oxidation of a nonaromatic hydrocarbon having at least fourcarbon atoms in a straight chain with molecular oxygen or a molecularoxygen containing gas in the vapor phase at a temperature from abut 300°C. to about 600° C. in the presence of a phosphorus-vanadium mixed oxidecatalyst wherein the catalyst is prepared by:(a) contacting atetravalent vanadium compound, dissolved in an aqueous, non-oxidizingacid medium, with crystalline diphosphoric acid to form aphosphorus-vanadium mixed oxide catalyst precursor; (b) crystallizingthe catalyst precursor from the catalyst precursor aqueous solution in acontrolled manner involving at least three cycles ofconcentrating/diluting wherein a fraction of the liquid from about 0.15to about 0.85 is removed during the concentrating step and water isadded during the diluting step in an amount sufficient to provide awater added/solvent removed volume ratio from about 0.10 to about 10.0,with the proviso that the final liquid/initial liquid volume ratio isfrom about 0.20 to about 2.0; (c) recovering the catalyst precursorcrystals; (d) drying the catalyst precursor; (e) forming the driedcatalyst precursor into structures, and (f) calcining the catalystprecursor structures at a temperature from about 300° C. to about 600°C.
 2. The process of claim 1 wherein the catalyst has aphosphorus/vanadium atom ratio from about 0.5 to about 2.0.
 3. Theprocess of claim 2 wherein the catalyst has a phosphorus/vanadium atomratio from about 0.95 to about 1.2.
 4. The process of claim 1 whereinthe tetravalent vanadium compound is selected from the group consistingof the halides, oxides, and oxyhalides of vanadium.
 5. The process ofclaim 4 wherein the tetravalent vanadium compound is a vanadiumoxyhalide.
 6. The process of claim 5 wherein the vanadium oxyhalide isvanadium oxydichloride.
 7. The process of claim 1 wherein thetetravalent vanadium compound is formed by the in situ reduction of apentavalent vanadium compound.
 8. The process of claim 7 wherein thepentavalent vanadium compound is selected from the group consisting ofhalides, oxides, and oxyhalides of vanadium.
 9. The process of claim 8wherein the pentavalent vanadium compound is a vanadium oxide.
 10. Theprocess of claim 9 wherein the vanadium oxide is vanadium pentoxide. 11.The process of claim 1 wherein the aqueous, non-oxidizing acid medium isconcentrated hydrochloric acid.
 12. The process of claim 1 wherein thecatalyst precursor crystals are recovered by filtration.
 13. The processof claim 1 wherein the catalyst precursor structures are tablets. 14.The process of claim 1 wherein the catalyst precursor calcinationtemperature is about 400° C.
 15. The process of claim 1 wherein thenonaromatic hydrocarbon is a saturated hydrocarbon.
 16. The process ofclaim 15 wherein the saturated hydrocarbon is n-butane.
 17. The processof claim 1 wherein the molecular oxygen-containing gas is air.
 18. Theprocess of claim 17 wherein the nonaromatic hydrocarbon-in-airconcentratation is from about one mole percent to about 10 mole percent.19. The process of claim 18 wherein the nonaromatic hydrocarbon-in-airconcentration is about 1.5 mole percent.
 20. The process of claim 1wherein the oxidation of the nonaromatic hydrocarbon with molecularoxygen or a molecular oxygen-containing gas is carried out at atemperature from about 325° C. to about 425° C.
 21. The process of claim1 wherein the phosphorus-vanadium mixed oxide catalyst is employed in afixed bed.
 22. A process for the production of maleic anhydride by theoxidation of n-butane with molecular oxygen or a molecular oxygencontaining gas in the vapor phase at a temperature from about 325° C. toabout 425° C. in the presence of a phosphorus-vanadium mixed oxidecatalyst having a phosphorus/vanadium atom ratio of about 0.95 to about1.2 wherein the catalyst is prepared by:(a) contacting vanadiumoxydichloride, dissolved in aqueous hydrochloric acid, with crystallinediphosphoric acid to form a phosphorus-vanadium mixed oxide catalystprecursor; (b) crystallizing the catalyst precursor from the catalystprecursor aqueous solution in a controlled manner involving at leastthree cycles of concentrating/diluting wherein a fraction of the liquidfrom about 0.15 to about 0.85 is removed during the concentrating stepand water is added during the diluting step in an amount sufficient toprovide a water added/solvent removed volume ratio from about 0.10 toabout 10.0, with the proviso that the final liquid/initial liquid volumeratio is from about 0.20 to about 2.0; (c) filtering the catalystprecursor crystals from the water-diluted catalyst precursor mixture;(d) drying the catalyst precursor; (e) forming the catalyst precursorinto structures; and (f) calcining the catalyst precursor structures ata temperature from about 400° C. to about 500° C. for a period of timefrom about four hours to about eight hours.
 23. The process of claim 22wherein the molecular oxygen-containing gas is air.
 24. The process ofclaim 23 wherein the n-butane-in-air concentration is from about onemole percent to about 10 mole percent.
 25. The process of claim 24wherein n-butane-in-air concentration is about 1.5 mole percent.
 26. Theprocess of claim 22 wherein the phosphorus-vanadium mixed oxide catalystis employed in a fixed bed.